Fuel tanks may be classified into motorcycle fuel tank, automobile fuel tank and general fuel tank according to their uses. Motorcycle fuel tank mainly employs steel tank, while automobile fuel tank generally uses plastic tank and steel tank. Because of the strong requirements of lightening and cost reduction of automobiles, plastic fuel tanks have been favored by automobile manufacturers for a time, which leads to loss of a considerable portion of the market for steel tanks. However, along with the sequential issues of environmental protection and safety regulations which are increasingly strict in the automobile industry, steel fuel tanks exhibit their advantages gradually. The main direction of development is pointed to materials for steel fuel tanks which are environmentally friendly, free of lead and chromium, easily processable and highly corrosion resistant. At present, the materials for steel fuel tanks used commercially are generally classified into three types, namely carbon steel, stainless steel and aluminum alloy, wherein coated products of cold rolled carbon steel plate are most widely used. There are many types of coated products of carbon steel, including those obtained by hot-dip galvanizing, hot-dip zinc-iron alloy coating, hot-dip aluminizing, hot-dip lead coating, hot-dip tin-zinc coating, zinc-nickel electroplating, zinc electroplating, etc. Nippon Steel Co., JFE, POSCO, etc, have set foot in the above products.
Nippon Steel Co. replaces the traditional lead-tin coated steel plate with a steel plate material having hot-dip tin-zinc coating for automobile fuel tanks. Although corrosion resistance and high permeation resistance are achieved, it is a steel plate with hot-dip tin-zinc coating.
Electrogalvanized and surface treated steel plates for fuel tanks are available from JFE under the names of GT and GP, wherein single-face zinc-nickel electroplating plus single-face special treatment are conducted for GT which is used for corrosion-resistant fuel tanks, while double-face electrogalvanizing plus double-face special treatment are conducted for GP which is used for highly corrosion-resistant fuel tanks.
Chinese patent application CN1277640A discloses a surface treated steel plate used for oil tanks and a process of making the same. The surface treated steel plate is obtained by a double coating/double baking process resulting in a chromate skin film and a resin coating on a zinc or zinc family alloy coating, wherein the chromate layer is formed by using trivalent chromium, phosphoric acid, fluoric acid, sulfuric acid, colloidal silica and an epoxy silane coupling agent as primary film forming agents, and baking at a metal temperature in the range of 120-250° C. for solidification; and the resin coating is formed by using a resin solution comprising the essential components of phenoxy resin, colloidal silica and melamine resin, and baking at a metal temperature in the range of 160-250° C. for solidification. This surface treated steel plate exhibits good corrosion resistance, chemical resistance, fuel resistance and coating adhesion. However, the double coating/double baking process is rather complex and expensive, and the surface treatment with chromate cannot meet the requirement of environmental protection that a chromium free material should be used.
Chinese patent application CN101346493A discloses a surface treated, chromium free steel plate used for oil tanks and a process of making the same. The surface treated steel plate is obtained by a double coating/double baking process resulting in a chromium free skin film and a resin coating on a zinc-based electroplated steel plate, wherein the chromium free layer is formed by using silicate, a silane coupling agent, a titanium compound, an binder resin and a phosphoric acid ester as primary film forming agents, and baking at a metal temperature in the range of 120-250° C. for solidification; and the resin coating is formed by using a solution comprising the essential components of phenoxy resin, melamine resin, silica, silicon dioxide, metal powder and a phosphoric acid ester, and baking at a metal temperature in the range of 190-250° C. for solidification. Although the requirement of environmental protection with respect to the material used for fuel tanks is taken into account in this invention, similar to Chinese patent application CN1277640A, a double coating/double baking process is used in the manufacture, and an even higher solidification temperature is used. Hence, there exist problems including process complexity, high demand on production equipments, high energy consumption, etc., which lead to high manufacture cost.
Chinese patent application CN102400076A discloses a hot-dip tin-zinc alloy coated steel plate for fuel tanks and a method of making the same. It is a zinc-tin coated product, and used for processing fuel tanks without surface treatment of the coating.